Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), Laterally Coupled Resonators Filters (LCRFs), and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric layer between a bottom (first) electrode and a top (second) electrode over a cavity. BAW resonators may be used in a wide variety of electronic applications and devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs operating at frequencies close to their fundamental resonance frequencies may be used as a key component of radio frequency (RF) filters and duplexers in mobile devices.
An acoustic resonator typically comprises a layer of piezoelectric material applied to a top surface of a bottom electrode, and a top plate electrode applied to a top surface of the piezoelectric material, resulting in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
Generally, a conventional FBAR designed to operate at approximately 3.6 GHz, for example, would have approximately 1800 Å thick top and bottom electrodes formed of tungsten (W), and an approximately 2600 Å thick piezoelectric layer for of aluminum nitride (AlN). The thickness of the piezoelectric layer is predominantly determined by the desired frequency of operation, but also by the desired electromechanical coupling coefficient Kt2. Within applicable limits, the electromechanical coupling coefficient Kt2 is proportional to thickness of the piezoelectric layer and inversely proportional to thicknesses of bottom and top electrodes. More specifically, the electromechanical coupling coefficient Kt2 is proportional to the fraction of acoustic energy stored in the piezoelectric layer and inversely proportional to the fraction of acoustic energy stored in the electrodes. This design, requiring a reasonable electromechanical coupling coefficient Kt2 (e.g., approximately 5.5 percent), has a number of problems operating at high frequencies (e.g., greater than or equal to about 3.5 GHz). For example, such an FBAR would be susceptible to electrostatic discharge (ESD) failures due to large electric fields, low-power failures due to small area, large perimeter-to-area loss due to small device area, and large series resistance Rs due to the presence of relatively thin metal top and bottom electrodes.